Coordinate positioning machine

ABSTRACT

A non-Cartesian coordinate positioning machine is provided that comprises an extendable leg assembly for positioning a component such as a measurement probe within a working volume of the machine, and a constraint member associated with the extendable leg assembly for providing a predetermined part of the extendable leg assembly with substantially a same orientation relative to gravity for a same position of the component within the working volume. In a preferred embodiment, the orientation relative to gravity is maintained substantially constant, so that a plane defined by the predetermined part is substantially aligned with gravity, as the component is moved around the working volume.

This is a Continuation of U.S. application Ser. No. 15/744,326, filedJan. 12, 2018, which is a U.S. National Phase of InternationalApplication No. PCT/GB2016/052392, filed Aug. 4, 2016, which claimspriority from GB 1513850.6 filed Aug. 5, 2015, the entire disclosures ofwhich is incorporated by reference.

The present invention relates to a coordinate positioning machine, andin particular to a non-Cartesian coordinate positioning machine such asa hexapod coordinate positioning machine. Coordinate positioningmachines include, for example, coordinate measuring machines (CMMs) andmachine tools.

As illustrated schematically in FIG. 1 of the accompanying drawings, anon-Cartesian coordinate positioning machine 1 generally comprises firstand second stages or platforms 2, 4 that are supported and movedrelative to each other by a plurality of telescopic or extendable legs 6provided between them. The extendable legs 6 are sometimes also referredto as struts or rams, and where there are six such extendable legs 6 (asillustrated in FIG. 1), the machine is commonly called a hexapod.

The extendable legs 6 are typically mounted on the platforms 2, 4 viaball joints 8, with each leg 6 either having its own ball joint 8 at oneor both ends thereof (as illustrated in FIG. 1), or sharing a ball joint8 with an adjacent leg 6 at one or both ends.

Various relative positions and orientations between the first and secondplatforms 2, 4 can be achieved by extending the legs 6 by differingamounts, as illustrated in FIG. 1 by arrows 7. The relative position andorientation at any instant is monitored by a plurality oflength-measuring transducers 10, for example with one transducer beingassociated with each extendable leg 6. The length-measuring transducermay comprise an encoder scale paired with a readhead.

One of the platforms 2, 4 is typically provided as part of a fixedstructure of the positioning machine 1, with the other of the platforms4, 2 moving 5, 3 relative to the fixed structure. A component (forexample a probe or a tool) can be mounted on the moving platform and aworkpiece mounted to the fixed structure, or vice versa, to enable anoperation to be performed on the workpiece (for example measuring,probing, or scanning in the case of a coordinate measuring machine, ormachining in the case of a machine tool).

For example, as illustrated in FIG. 1, the lower platform 4 is fixed andthe upper platform 2 is moveable, with a workpiece 12 mounted on thelower platform 4 and a probe component 14 mounted on the upper platform2. A working volume 9 is defined between the upper platform 2 and thelower platform 4, with the probe component 14 being positioned in theworking volume 9 by operation of the extendible legs 6. Although avertical arrow 3 is shown to indicate movement, with appropriate controlof the various legs 6 the platform 2 will of course also be moveablehorizontally and could also be tiltable.

Alternatively, the upper platform 2 could be fixed and the lowerplatform 4 moveable, with a probe mounted to a lower surface of thelower platform 4 and a workpiece mounted to a part of the fixedstructure below that, so that the working volume (or operating volume)of the machine is below the lower platform 4 rather than between theupper and lower platforms 2, 4.

Various types of non-Cartesian coordinate positioning machine aredescribed in more detail in WO 91/03145, WO 95/14905, WO 95/20747, WO92/17313, WO 03/006837, WO 2004/063579, WO 2007/144603, WO 2007/144573,WO 2007/144585, WO 2007/144602 and WO 2007/144587.

For example, WO 91/03145 describes a hexapod machine tool comprising anupper, moveable, platform that is attached to a base by six hydraulicextendable legs, similar in principle to that illustrated in FIG. 1described above. The extendable legs are attached to the base andmoveable platform via ball joints. The extendable legs are hydraulic andcomprise a piston rod that is moveable within a cylinder. The amount ofleg extension is measured by mounting a magnetic scale to the cylinderand a suitable readhead on the piston rod. Extension of the leg thuscauses the scale to move past the readhead thereby allowing the lengthof the leg to be measured. A computer controller acts to set the lengthof each leg to provide the required platform movement.

As with any metrology apparatus, positional accuracy and repeatabilityare important, and various schemes have previously been proposed inorder to improve positional accuracy and repeatability in anon-Cartesian coordinate positioning machine.

For example, WO 2007/144573 recognises that load forces that occur inthe apparatus during use may introduce distortions into metrologyelements of the apparatus, thereby leading to positional inaccuracies.Therefore, WO 2007/144573 describes an improvement to WO 91/03145, inwhich a position measurement apparatus is provided with a metrologyframe that is separate from the thrust frame. Any load forces that mayoccur in the load-bearing structure are thereby not passed through tothe metrology structure, thus preventing any substantial distortion ofthe metrology frame and thereby ensuring measurement accuracy is notdegraded. The separation of the load-bearing structure from themetrology structure applies to each of the six extendable legs, witheach leg being provided with have a load-bearing outer structure and ametrology inner structure, with the metrology structure of the legsbeing mechanically isolated from the load-bearing structure. This isapparent particularly from FIG. 3 of WO 2007/144573.

WO 95/14905 describes a variant of the above described hexapod apparatusin which the length of each extendable leg is measuredinterferometrically.

According to a first aspect of the present invention, there is provideda non-Cartesian coordinate positioning machine comprising: an extendableleg assembly for positioning a component within a working volume of themachine; and a constraint member associated with the extendable legassembly for providing a predetermined part of the extendable legassembly with substantially a same orientation relative to gravity for asame position of the component in the working volume.

The component may comprise a metrology component or metrologyinstrument, such as a measurement probe. The predetermined part maycomprise a metrology element, such as a metrology element that is usedfor measuring a separation between ends of the extendable leg assembly,or some other length associated with the extendable leg assembly.

One such example of a metrology element is an encoder scale. Where themetrology element is an encoder scale, an embodiment of the presentinvention can be used to prevent or at least reduce movements of theextendable leg assembly which may cause the encoder scale to be subjectto stresses and strains that are likely to affect metrology measurementsusing the encoder scale.

The benefit of the invention in relation to the specific example of anencoder scale will now be described in more detail.

In prior art hexapod apparatus of the type described further above, theextendable legs typically comprise pistons or electronic drive motorsthat act to extend and retract each leg as required. Typically, each legalso comprises some means for measuring the amount of leg extension. Forexample, as mentioned above, WO 91/03145 describes hydraulic extendablelegs that comprise a piston rod moveable within a cylinder. The amountof leg extension is measured by mounting a magnetic scale to thecylinder and a suitable readhead on the piston rod. Extension orretraction of the leg thus causes the scale to move past the readheadthereby allowing the length of the leg to be determined.

The present applicant has appreciated a key disadvantage of arrangementsof the type described in WO 91/03145. Since the scale is affixed to anouter surface of the cylinder, bending of the cylinder due to gravity islikely to cause distortion of the scale. Furthermore, the amount andtype of scale distortion will depend on exactly where and in whatorientation the scale is affixed.

This problem is illustrated in FIGS. 2 and 3 of the accompanyingdrawings. If the scale 10 is mounted to an upper surface of theextendable leg 6, as illustrated in FIG. 2A, bending due to gravity willcause the upper surface (and scale) to contract, as illustrated in FIG.2B. The scale markings will, as a result, be more closely spaced,leading to errors in the measurement of length (a higher amount ofrelative movement between telescoping sections of the extendable legwill be measured than is actually the case).

If the scale 10 is instead mounted to a lower surface of the extendableleg 6, as illustrated in FIG. 3A, bending due to gravity will cause thelower surface (and scale) to stretch, as illustrated in FIG. 3B. Thescale markings will, as a result, be more widely spaced, leading toerrors in the measurement of length (a lower amount of relative movementbetween telescoping sections of the extendable leg will be measured thanis actually the case).

To make matters worse, the orientation of the encoder scale 10 willchange as the extendible legs 6 are adjusted to move one or bothplatforms 2, 4, and thereby the component (e.g. probe 14), around theworking volume.

Although the scheme described in WO 95/14905 reduces some gravitationaleffects by providing measurement down the centre of the member (theneutral axis), and WO 2007/144573 does so by isolating the load bearingstructure from the metrology structure, such arrangements arecomplicated and expensive to implement.

In contrast, an extendable leg assembly embodying the present inventionoffers a relatively straightforward and inexpensive solution to theabove problem by use of a constraint member to constrain or control theorientation of the encoder scale. The constraint member can be used toprovide the encoder scale (or a plane defined by the encoder scale) withsubstantially the same orientation relative to gravity for all positionsof the component within the working volume. In particular, theconstraint member can be arranged to keep the encoder scale in avertical plane as the component is moved around the working volume. Indoing so, the problem caused by bending of the extendible leg isovercome or at least significantly reduced. This is because aside-facing surface will experience some contraction at the top and somestretching at the bottom, with a neutral axis that is substantiallyunaffected. The encoder scale can be arranged along the neutral axis.Although the encoder scale has a finite area, and will be affected offthe neutral axis, the overall effect is neutral. This overcomes theproblem described with reference to FIG. 3.

In the above-described scenario, the constraint member is arranged toprovide the predetermined part (the encoder scale or the plane of theencoder scale) with a substantially constant orientation relative togravity. However, in other embodiments of the present invention thepredetermined part need not necessarily have the same (or a fixed or aconstant) orientation relative to gravity for all positions of thecomponent in the working volume; it is merely required that thepredetermined part has the same orientation relative to gravity for thesame position.

In this respect, a varying (but repeatable) orientation relative togravity is beneficial, for example, where the coordinate positioningmachine is used as a comparator rather than as a coordinate measuringmachine. With a comparator, measurements of a part are compared withmeasurements of a reference part in the same position and orientation inthe working volume. By ensuring that the predetermined part has the sameorientation relative to gravity for the same position, the gravitationaleffect described above with reference to FIG. 3 will be the same foreach position of the probe within the working volume and willeffectively cancel when a comparison (or difference) is made.

Therefore, for a comparator it is more important that the orientation ofthe encoder scale relative to gravity is repeatable than that theorientation of the encoder scale relative to gravity is fixed. On theother hand, where the coordinate positioning machine is a coordinatemeasuring machine, it is more important that the orientation of thepredetermined part (e.g. encoder scale) is fixed relative to gravity(e.g. aligned with gravity), since absolute measurement accuracy isimportant. In being fixed, the orientation is also by definitionrepeatable.

The use of a constraint member in an embodiment of the present inventiontherefore ensures that, as the component is moved around the workingvolume from position to position, the predetermined part of theextendable leg assembly (e.g. the encoder scale) will have anorientation relative to gravity that is repeatable for each position. Inother words, if the component is moved from a first position to a secondposition and then back to the first position, the predetermined partwill have substantially the same orientation relative to gravity in thefirst position both before and after the move to the second position.The constraint member therefore ensures that the predetermined part hassubstantially a same orientation relative to gravity for a same positionof the component at different times as the component is moved around theworking volume.

The orientation may be that of a plane defined by the predeterminedpart, for example a planar or plane-like surface of the predeterminedpart. The orientation of a plane may conveniently be defined by thenormal to the plane. In this respect, a plane can be defined by a point(which sets the position of the plane) and a normal vector (which setsthe orientation of the plane). Therefore, stating herein that a plane isaligned with gravity is equivalent to stating that a normal to the planeis normal to the direction of gravity, or that the plane is oriented(its normal vector is pointing) at right angles to gravity.

It will be appreciated that the present invention will provide somebenefit even where the orientation relative to gravity is not exactlyfixed or not exactly repeatable. Some differences in the orientation fordifferent visits of the component to the same position may be allowed,where those differences are acceptable or tolerable for the applicationconcerned (e.g. the measuring accuracy required). In other words, some‘play’ or tolerance in the constraint may be generally acceptable. It ispreferable that the orientations relative to gravity for a same positionare within 10% of each other (maximum value minus minimum value dividedby minimum value, expressed as a percentage), more preferably within 5%of each other, more preferably within 2% of each other, more preferablywithin 1% of each other; what is acceptable will depend on theapplication and the accuracy required.

It will be appreciated that the present invention is also applicablewhere the predetermined part is made to have substantially the sameorientation relative to a reference direction other than gravity, forexample a reference direction defined by predetermined features of themachine rather than by an external gravitational field. For example, thepredetermined features of the machine may be ones that would be orientedsubstantially vertically in use, such as a support or frame member. Or,where there is another force acting on the system, the referencedirection may be the direction of that force. For example, there may bean application of the invention where the reference direction issubstantially horizontal.

The predetermined part may comprise a measurement device, such as alength-measuring device, or at least part thereof. The predeterminedpart may interact (or cooperate) at a measurement location with afurther part of the machine to provide a measurement relating to theextendable leg assembly, or at least a signal from which such ameasurement may be derived. The predetermined part and further part maytogether form a transducer. The measurement may be a length of orassociated with the extendable leg assembly. Such a measurement may beused to determine the position of the component in the working volume.The predetermined part may comprise a scale, with the further partcomprising a scale reader. The further part may be a part of theextendible leg assembly.

The extendable leg assembly may comprise first and second members whichmove relative to one another when the extendable leg assembly changeslength. For example, the first and second members may slide over or pastone another, for example telescopically. The first and second membersmay together form an elongate member of the extendable leg assembly. Thepredetermined part of the extendable leg assembly may be provided on (orby or form part of) the first member, with the further part provided on(or by or forming part of) the second member.

The extendable leg assembly may be supported (or held) in the machine byat least one support, such as at an end (or joint) of the extendable legassembly. The extendable leg assembly may be provided between first andsecond platforms of the machine, with the first and second platformsbeing positioned relative to each other by the extendable leg assembly.The component may be attached to one of the first and second platforms.One of the first and second platforms may be fixed (stationary), withthe component being attached to the other of the first and secondplatforms. A weight of the moving platform may be at least partially(and preferably substantially completely) supported by a counterbalancearrangement. The term platform is a broad term to describe any type ofstructure, and is not intended to imply any limitations as to form andshape.

The measurement location may be spaced away or apart from the or eachsupport. With such an arrangement, the extendable leg assembly is atleast to some extent unsupported (or self-supporting) at the measurementlocation, and therefore subject at least to some extent togravitationally-induced bending at the measurement location (andvulnerable, in the absence of the present invention, to the types oftechnical problem described above with reference to FIG. 3, due tostresses caused by bending of the elongate member due to gravity). Thespacing between the measurement location and at least one of the atleast one support may vary as the component moves around the workingvolume (and as the length of the extendable leg assembly varies).

The extendable leg assembly may be supported (e.g. held) by first andsecond supports at first and second positions respectively, for exampleat first and second ends of the extendable leg assembly. The measurementlocation may be arranged between the first and second positions. Themeasurement location may be spaced apart from both of the first andsecond positions. The spacing between the measurement location and atleast one of the first and second positions may vary as the componentmoves around the working volume (and as the length of the extendable legassembly varies).

For at least one position of the component in the working volume (or forat least one extension or length of the extendible leg assembly, such asfully extended), the measurement location may be spaced apart from atleast a closest one of the at least one support (e.g. spaced apart fromone or both of the first and second positions) by at least five times awidth (or diameter) of an elongate member on or by which thepredetermined part is provided, or at least 10 times that width, or atleast 25 times that width, or at least 50 times that width. For at leastone position of the component in the working volume (or for at least oneextension or length of the extendible leg assembly, such as fullyextended), the measurement location may be spaced apart from at least aclosest one of the at least one support (e.g. spaced apart from one orboth of the first and second positions) by at least 10% of an overalllength of the extendible member (or extendible leg assembly) for thatposition of the component in the working volume, or at least a quarterof the overall length, or at least a third of the overall length, orapproximately (e.g. within 10% of) half of the overall length.

For example, the width or diameter of an elongate member of theextendable leg assembly, on or by which the predetermined part isprovided, may be around 8 mm. The elongate member may be enclosed in ashell, the outside diameter of which may be around 35 mm. The length ofthe extendible leg assembly may vary between around 500 mm long andaround 850 mm as the component moves around the working volume. It is tobe understood that these values are purely by way of example and notlimiting on the invention in any way.

It is to be appreciated that, where it is described herein that thecomponent is moved to a particular position in the working volume, thiscould be by way of drive means provided by the coordinate positioningmachine, for example as part of or at least associated with theextendible leg assembly itself, or this could be by way of some externalinfluence, for example manual positioning of the component by anoperator.

Also, where it is described that the extendable leg assembly is forpositioning the component within the working volume, this is to beunderstood as meaning either setting the position of the componentwithin the working volume (by actively moving the component to thatposition) or determining the position of the component within theworking volume (the component having been moved to that position bywhatever means), or a combination of these. In either case, positioningthe component within the working volume is associated with moving thecomponent around the working volume, and is not intended to cover merelydetermining the position of a static component (e.g. a workpiece) placedwithin the working volume.

The component may be attached directly or indirectly to and/or move withan end of the extendable leg assembly, so that the component can bemoved around the working volume by operation of the extendable legassembly. The component may be a measurement probe, or a part thereof(such as a stylus or a stylus tip). The component may be a tool, or apart thereof, such as a tool typically found in a machine tool forshaping or machining metal or other rigid materials. The component caneven be considered to be a part, for example a moveable end, of theextendable leg assembly itself, for example defined by a ball joint atthat end.

By a same position of the component within the working volume it ismeant that the component has substantially a same position andorientation (defined by all six degrees of freedom). In the sameposition, the component will occupy substantially the same volume.

Furthermore, the term ‘working volume’ is intended to mean only thatpart of the working volume over which the present invention has effect.For example, the constraint described may not operate over the entirevolume in which the component is physically able to move via theextendable leg assembly, but may operate (or may operate effectively)only over a part thereof. The term ‘working volume’ is to be construedaccordingly, and should be read as ‘at least part of the working volume’where appropriate.

The constraint member may be adapted to constrain rotation of thepredetermined part relative to a plane defined by the constraint memberwhen the extendable leg assembly is arranged in the machine.

The extendable leg assembly may comprise an extendible elongate member,with the predetermined part being moveable with the extendible elongatemember. The predetermined part may be attached to (or affixed to, orformed as part of, or defined by) the elongate member. The predeterminedpart may be attached at or near a centre of the surface of the elongatemember, or along a line running down a middle of the surface.

The constraint member may be attachable to the elongate member and to afurther member of the coordinate positioning machine, wherein theconstraint member is adapted to constrain rotation of the predeterminedpart relative to the plane defined by the constraint member when theconstraint member is attached to the elongate member and to the furthermember.

The constraint member may be adapted to constrain motion of the elongatemember relative to a plane defined by the constraint member, when theconstraint member is attached to the elongate member and to the furthermember.

The predetermined part may be of a type that is affected by theorientation of the elongate member relative to the plane defined by theconstraint member. A surface of the elongate member to which the elementis attached may be arranged to be parallel to the plane defined by theconstraint member. If the constraint member is arranged in thecoordinate positioning machine such that the plane defined by theconstraint member is substantially vertical (or substantially alignedwith the gravity), and if the surface of the elongate member to whichthe element is affixed is arranged to be parallel to that plane, thenthe constraint member will act to keep the encoder scale in a verticalplane.

Aside from the benefit described above of overcoming the technicalissues described with reference to FIG. 3, the provision of a constraintaccording to an embodiment of the present invention can also bebeneficial in other ways. For example, there may be an element attachedto (or affixed to, or formed as part of, or defined by) the extendableleg assembly, where that element may have an effect on the orientationof the extendable leg assembly (or of an elongate member forming part ofthe extendable leg assembly) about the longitudinal axis and/or relativeto the plane.

For example, the element may comprise a mechanical element, such as adrive element that is used for changing a separation between ends of theelongate member or a length associated with the extendable leg assembly.The drive element may be a motor for extending and retracting theextendable leg assembly.

Such a mechanical element may be relatively weighty, and if located offthe longitudinal axis of the elongate member, which it typically wouldbe, it will tend to cause rotation of the elongate member about itslongitudinal axis. Such rotation can lead to problems, particularlywhere a joint is shared between adjacent leg assemblies such that theleg assemblies are situated in close proximity at the joint; in thissituation, any rotation of the elongate member about its longitudinalaxis may cause the adjacent leg assemblies to clash with one another,which in turn can cause the leg assembly to lift slightly off the joint.This is likely to lead to measurement errors, or could even cause theleg assembly to come off the joint completely.

Therefore, the use of a constraint member as an anti-rotation device insuch a situation is beneficial since the constraint member will preventsuch rotation about the longitudinal axis, or at least reduce suchrotation to a sufficient extent that there is no longer a significantrisk of the ends of adjacent leg assemblies clashing.

In such a situation, the constraint member may be arranged in themachine with the plane defined by the constraint member beingsubstantially aligned with (e.g. parallel to or coincident with) a planedefined by the elongate member and an adjacent elongate member.

Rotation of the elongate member about a longitudinal axis of theelongate member can be considered to be a first rotational degree offreedom of the elongate member. This degree of freedom is constrained.The constraint member may also be adapted to constrain rotation of theelongate member out of the plane. This can be considered to be a secondrotational degree of freedom of the elongate member. The constraintmember may also be adapted to allow rotation of the elongate memberwithin the plane. This can be considered to be a third rotational degreeof freedom of the elongate member.

It can be considered that the constraint member is adapted to constrainrotation, relative to the plane defined by the constraint member, of avector that is transverse (for example perpendicular) to a longitudinalaxis of the elongate member.

The constraint member may be adapted to maintain a substantiallyconstant angle between the plane defined by the constraint member and avector that is transverse to the longitudinal axis of the elongatemember. The vector is fixed relative to the elongate member (and somoves and rotates with the elongate member). Such a constrainteffectively prevents (or at least reduces) rotation of the elongatemember about the longitudinal axis of the elongate member (the firstdegree of freedom mentioned above).

The transverse vector may also be transverse to the plane defined by theconstraint (as well as being transverse to the longitudinal axis of theelongate member). In such a case, the constraint also effectivelyamounts to preventing (or at least reducing) rotation of the elongatemember in the second degree of freedom mentioned above.

However, such a constraint allows rotation of the elongate member in thethird degree of freedom, since in doing so the angle between the planeand the transverse vector will remain constant.

The constraint member may be adapted to allow rotation of the planeabout an attachment axis defined by an attachment feature which is usedto attach the constraint member to the further member. The attachmentaxis may lie in or parallel to the plane defined by the constraintmember. The attachment axis may be arranged in use to be aligned with orparallel to gravity.

It is to be noted that a first direction is considered to be transverseto a second direction if there is a non-zero angle between the first andsecond directions. A particular case is where the first direction isperpendicular to the second direction.

Furthermore, by ‘constraining rotation’ it is meant that such rotationis substantially prevented or at least reduced by the action of theconstraint member.

The constraint member may be adapted to allow rotation of the planeabout an attachment axis defined by an attachment feature which is usedto attach the constraint member to the further member. The attachmentaxis may lie in or parallel to the plane. The attachment axis may bearranged in use to be aligned with or parallel to gravity.

The constraint member may be attached to the elongate member or it maybe provided separate to the elongate member, for example in a kit ofparts, ready for attachment to the elongate member.

Similarly, the constraint member may be attached to the further memberor it may be provided separate to the further member, for example in akit of parts, ready for attachment to the further member.

The constraint member may be adapted so as to be readily and easilyattachable and detachable to and from one or both of the elongate memberand the further member.

The elongate member may be supported within the machine in a manner thatwould, in the absence of the constraint member, allow rotation of theelongate member about its longitudinal axis.

The elongate member may be supported at one or both ends by a pivotjoint, such as a ball joint.

The longitudinal axis of the elongate member would typically beconsidered to lie between the joints at either end of the elongatemember.

For the pivot joint used in an embodiment of the present invention, abearing arrangement may be provided at a first end of the elongatemember, with the bearing arrangement providing three contact points, orsubstantially point-like contact surfaces, for bearing against an atleast part spherical bearing surface provided on the machine, where aplane defined by the contact points or areas is substantiallyperpendicular to a longitudinal axis of the elongate member. The bearingarrangement thereby provides a kinematic or at least pseudo-kinematiccoupling between the elongate member and the machine. The at least partspherical bearing surface may be provided by a ball, or part thereof,fixed in relation to the machine. Such a bearing arrangement may beprovided at both ends of the elongate member. The three contact pointsor areas may be provided by three at least part spherical surfaces, suchas three balls, each of which may be smaller than the at least partspherical surface (or ball) associated with the machine.

According to a further aspect of the present invention, there isprovided an extendable leg assembly and associated constraint membersuitable for use in a coordinate positioning machine according to thefirst aspect of the present invention.

According to a further aspect of the present invention, there isprovided a constraint member for a non-Cartesian coordinate positioningmachine having an extendable leg assembly for positioning a componentwithin a working volume of the machine, wherein the constraint member isadapted to provide a predetermined part of the extendable leg assemblywith substantially a same orientation relative to gravity for a sameposition of the component within the working volume.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising such aconstraint member.

According to a further aspect of the present invention, there isprovided a constraint member for constraining movement of an extendableleg assembly in a non-Cartesian coordinate positioning machine, whereinthe constraint member is attachable to an elongate member of theextendable leg assembly and to a further member of the coordinatepositioning machine, and wherein the constraint member defines a planeand is adapted to constrain rotation of the elongate member relative tothat plane when the constraint member is attached to the elongate memberand to the further member. All of the subsidiary features mentionedabove in connection with the constraint member in the first aspect ofthe present invention apply to this aspect too.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising an elongatemember and a constraint member attachable to the elongate member and toa further member of the coordinate positioning machine, wherein theconstraint member is adapted to constrain rotation of the elongatemember around a longitudinal axis of the elongate member when theconstraint member is attached to the elongate member and to the furthermember. The constraint member may be adapted to constrain rotation ofthe elongate member relative to a plane defined by the constraint memberwhen the constraint member is attached to the elongate member and to thefurther member. All of the subsidiary features mentioned above inconnection with the first aspect of the present invention apply to thisaspect too.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising an elongatemember and a constraint member attachable to the elongate member and toa further member of the coordinate positioning machine, wherein theconstraint member is adapted to constrain rotation of the elongatemember relative to an axis and/or a plane defined by the constraintmember when the constraint member is attached to the elongate member andto the further member. All of the subsidiary features mentioned above inconnection with the first aspect of the present invention apply to thisaspect too.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising an elongatemember having a bearing arrangement provided at a first end thereof,with the bearing arrangement providing three contact points, orsubstantially point-like contact surfaces, for bearing against an atleast part spherical bearing surface provided on the machine, where aplane defined by the contact points or areas is substantiallyperpendicular to a longitudinal axis of the elongate member. Preferablefeatures of the bearing arrangement are mentioned above in relation tofirst aspect of the present invention.

According to a further aspect of the present invention, there isprovided a non-Cartesian coordinate positioning machine comprising anextendable leg assembly for positioning a component within a workingvolume of the machine, and a constraint member associated with theextendable leg assembly for providing a predetermined part of theextendable leg assembly with substantially a same orientation relativeto gravity for a same position of the extendible leg assembly within themachine.

According to a further aspect of the present invention, there isprovided a non-Cartesian coordinate positioning machine comprising anextendable leg assembly for positioning a moveable component within aworking volume of the machine, and a constraint member associated withthe extendable leg assembly for providing a predetermined part of theextendable leg assembly with substantially a same orientation relativeto gravity for a same relative positioning of ends of the extendible legassembly.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising an elongatemember and a constraint member, wherein the constraint member is adaptedto constrain rotation of the elongate member relative to a plane definedby the constraint member when the extendable leg assembly is arranged inthe machine.

According to a further aspect of the present invention, there isprovided an extendable leg assembly for a non-Cartesian coordinatepositioning machine, the extendable leg assembly comprising an elongatemember and a constraint member for maintaining a predetermined part orsurface of the elongate member substantially aligned in use withgravity.

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1, discussed hereinbefore, is schematic illustration of a hexapodnon-Cartesian coordinate positioning machine, having six extendablelegs;

FIGS. 2A and 2B illustrate a problem associated with having an encoderscale affixed to an upper surface of an extendable leg of thenon-Cartesian coordinate positioning machine of FIG. 1;

FIGS. 3A and 3B illustrate a problem associated with having an encoderscale affixed to a lower surface of an extendable leg of thenon-Cartesian coordinate positioning machine of FIG. 1;

FIG. 4 is an overall view of a non-Cartesian coordinate positioningmachine comprising extendable leg assemblies according to an embodimentof the present invention;

FIG. 5 is a close-up view of part of the machine of FIG. 4, showing inmore detail the connection between the extendable leg assemblies and thelower platform of the machine;

FIG. 6 is a close-up view of part of the machine of FIG. 4, showing theconnection between the extendable leg assemblies and the upper platformof the machine, and in particular showing constraint members associatedwith two of the extendable leg assemblies;

FIG. 7 shows in more detail a constraint member according to anembodiment of the present invention;

FIGS. 8A to 8D show a constraint member according to an embodiment ofthe present invention from various different angles;

FIGS. 9A to 9D show an extendable leg assembly according to anembodiment of the present invention from various different angles, andin particular illustrating the relative positions of and therelationship between the constraint member and the encoder scale affixedto an elongate member of the extendable leg assembly;

FIG. 10 illustrates in more detail the relative positions of and therelationship between the constraint member and the encoder scale affixedto the elongate member of the extendable leg assembly, and the manner inwhich motion of the elongate member is constrained by the constraintmember;

FIG. 11 is similar to FIG. 10 but viewed from a different angle;

FIGS. 12A to 12C are schematic illustrations of the elongate member ofthe extendible leg assembly, showing in more detail how the scaleinteracts with a scale reader;

FIG. 13 illustrated another type of constraint suitable for use in anembodiment of the present invention;

FIG. 14 illustrates the use of a constraint member in another embodimentof the present invention, in which the constraint member is providedbetween two extendable legs of a non-Cartesian coordinate positioningmachine;

FIG. 15 is a close-up view of the constraint member of FIG. 14;

FIGS. 16A and 16B illustrate an alternative to the constraint member ofFIG. 14, with FIG. 16A showing an angled view and FIG. 16B showing a topview; and

FIGS. 17A and 17B illustrate an alternative to the constraint member ofFIG. 14, with FIG. 17A showing an angled view and FIG. 17B showing a topview.

FIG. 4 provides an overall view of a non-Cartesian coordinatepositioning machine 100 embodying the present invention. Thenon-Cartesian coordinate positioning machine 100 is similar in principleto that described above with reference to FIG. 1. However, a keydifference is the use of constraint members 50 in the non-Cartesiancoordinate positioning machine 100 to address the above-mentionedtechnical problems associated with known non-Cartesian coordinatepositioning machines.

The non-Cartesian coordinate positioning machine 100 illustrated in FIG.4 comprises six extendable leg assemblies 60, generally of the sameconstruction, arranged between an upper platform 20 and a lower platform40. Each of the six extendable leg assemblies 60 comprises an upper tube62 and a lower tube 64, with the upper tube 62 sliding telescopicallywithin the lower tube 64.

With the particular example illustrated in FIG. 4, the upper platform 20is fixed and the lower platform 40 is moveable relative to the upperplatform by operation of the six extendable leg assemblies 60, with aprobe 14 being mounted to a lower surface of the lower platform 40. Inthis configuration, a workpiece (not illustrated) would be mounted to apart of the fixed structure of the machine 100 below the lower platform40, so that the working volume of the machine 100 is below the lowerplatform 40 rather than between the upper and lower platforms 20, 40. Aweight of the lower (moving) platform 40 would typically be supported atleast partially by a counterbalance arrangement (not shown).

As with the machine of FIG. 1, the extendable leg assemblies 60 are forpositioning a component (in the example illustrated in FIG. 4, thecomponent is the probe 14, or at least part a specific part of the probe14 such as the tip of the probe 14) within the working volume of themachine. Constraint members 50 associated respectively with theextendable leg assemblies 60 are for providing a predetermined part ofthe extendable leg assembly (in this example, an encoder scale, to bedescribed further below) with substantially a same orientation relativeto gravity for a same position of the component within the workingvolume.

Upper and lowers ends of each extendable leg assembly 60 are connectedrespectively to the upper platform 20 and lower platform 40 viaindividual ball joints 80. The lower ball joints 80 for the front-mosttwo of the extendable leg assemblies 60 are just visible in FIG. 4, andshown in more detail in FIG. 5. The lower ball joints 80 are supportedby support blocks 42 of the lower platform 40, while the upper platform20 is supported on the upper ball joints 80 via support blocks 22 of theupper platform 20; the connection between the upper platform 20 and theextendable leg assemblies 60 is illustrated in more detail in FIG. 6.

The upper and lower tubes 62, 64 of each extendable leg assembly 60enclose an elongate member 66, shown in dotted outline in one of theextendable leg assemblies of FIG. 4, with an encoder scale 10 affixed tothe elongate member 66. The elongate member 66 is itself extendable, forexample by way of a telescopic arrangement. Each elongate member 66extends from its upper joint 80 to its lower joint 80, and it is therespective lengths of the elongate members 66 that determine the precisepositioning and orientation of the lower platform 40 (and therefore theprobe 14). It is therefore the length of the elongate members 66 thatmust be measured precisely during a measuring or scanning operation on aworkpiece in order to determine the precise location of the tip of thestylus when it is contact with the workpiece surface. Operation of anextendible elongate member 66 is described in more detail below withreference to FIG. 12.

At the upper end, each extendable leg assembly 60 is provided (orassociated) with a constraint member 50, which is attached to theelongate member 66 of the extendable leg assembly 60 and to a furthermember (the support block 22) provided on the upper platform 20. Theconstraint member 50 effectively ‘ties’ the elongate member 66 to theupper platform 20 in order to prevent (or at least reduce) undesiredrotation of the elongate member 66 about its longitudinal axis. Theconstruction and operation of the constraint member 50 will be describedin more detail below with reference to FIGS. 6 to 11.

FIG. 5 is a close-up view showing the connection between the extendableleg assemblies 60 and the lower platform 40 in more detail. As is shownparticularly in the zoomed view to the bottom of FIG. 5, three balls 84are provided in a triangular arrangement at the lower end of theelongate member 66, with the plane of the triangular arrangement beingsubstantially perpendicular to the longitudinal axis of the elongatemember 66. The support block 42 is provided with a larger, fixed, ball82 which acts to support the three balls 84 on the end of the elongatemember 66, with the larger ball 82 nestling within the smaller balls 84.

The use of three balls 84 means that coupling is kinematic in nature, orat least pseudo-kinematic, with three points of contact (or at leastthree small contact areas that approximate three points of contact). Theweight of the elements above the three balls 84 will naturally force theelongate member 66 into a repeatable position relative to the ball 82,with no over constraint, and thereby form a joint that is particularlysuitable for a metrology instrument where repeatability is important. Itis not essential to use balls 84, and instead a bearing arrangement canbe used that provides three points of contact (or small contact areas)which effectively form a kinematic cup or cone. It will also beappreciated that, where balls are used, they need not be complete ballsbut need only be at least part spherical in the areas that are to beused as bearing surfaces.

The ball joint arrangement shown in FIG. 5 has certain advantages overother known ball joint arrangements. In other arrangements, a singleball is provided at the end of the elongate member, and is supported bya fixed cone or cup arrangement. Alternatively, a ring having threepoint contacts engages with a fixed ball, but from the side. Anadvantage of an arrangement as shown in the present application is thatthe elongate member 66 is securely held on the ball 82 at a wider rangeof angles, providing a wide range of angular motion for the elongatemember 66 around the ball 82. With other known ball joint arrangements,the motion can be more restricted and/or less secure, with the elongatemember potentially being more prone to disengagement from the joint.

The pivot joint arrangement of FIG. 5 relates to an aspect of thepresent invention that is independent of the aspect which relates to theuse of constraint members 50, and it is to be emphasised that use of adifferent type of joint is perfectly feasible in conjunction with theconstraint members 50.

The constraint member 50 will now be described in more detail withreference to FIGS. 6 to 8. FIG. 6 is a close-up view showing in moredetail the connection between the extendable leg assemblies 60 and theupper platform 20 of the machine 100, and in particular showing in moredetail the constraint members 50 associated with two of the extendableleg assemblies 60 of FIG. 4. FIG. 7 shows one of the constraint members50 when detached from the support block 22, while FIGS. 8A to 8D show aconstraint member 50 from various angles (again, detached from thesupport block 22). In particular, FIG. 8A shows a front view of theconstraint member 50, with the ball joint 82, 84 in view, FIG. 8B showsa rear view, FIG. 8C shows a perspective view, and FIG. 8D shows a sideview.

The constraint member 50 can generally be described as a multi-part ormulti-section hinge. The illustrated example comprises four parts orsections 52, 54, 56 and 58 connected by rotary joints or knuckles 53, 55and 57 having substantially parallel axes of rotation. The joints 53, 55and 57 may be of a standard pin and bearing construction, and may useball bearings in order to reduce the effect of friction.

With the four parts 52, 54, 56 and 58 connected by joints 53, 55 and 57having substantially parallel rotation axes as illustrated, movement ofthe parts 52, 54, 56 and 58 relative to one is restricted to a movementin a plane, with the plane being perpendicular to the rotation axes ofthe joints 53, 55 and 57. In this way the constraint member 50 defines aplane, which will be described in more detail below with reference toFIGS. 10 and 11.

The upper-most part 58 is connected to the support block 22 via a rotaryjoint 59 having a rotation axis 59A (see FIG. 7) that is orientedsubstantially perpendicular to the rotation axes of the rotary joints53, 55, 57 (for example the rotation axis of joint 55 is marked as 55Ain FIG. 7). This allows for rotation of the constraint member 50 aboutthe axis 59A of joint 59 when the constraint member 50 is connected tothe upper platform 20, and thereby allows rotation about the axis 59A ofthe plane defined by the constraint member 50.

FIGS. 9A to 9D show the same four views of the constraint member 50 asare illustrated in FIGS. 8A to 8D respectively, but in FIGS. 9A to 9Dthe view is widened to include the elongate member 66 down to theopposite end. FIGS. 9A to 9D also illustrate the extendable leg assembly60 “opened up” in part to show the elongate member 66, and in particularthe positioning of a predetermined part of the extendable leg assembly60 whose orientation is to be controlled or constrained relative togravity. The predetermined part is an encoder scale 10, which is usedfor measuring the length of the elongate member 66 in conjunction with ascale reader or readhead (not shown), relative to the constraint member50. The scale 10 and scale reader are described in more detail belowwith reference to FIG. 12.

FIGS. 10 and 11 illustrate the relative positioning of the encoder scale10 and constraint member 50 in more detail. FIG. 10 is a side viewcorresponding to that of FIG. 9D, while FIG. 11 is a perspective viewcorresponding to that of FIG. 9C. As mentioned above, the constraintmember 50 defines a plane 51, as illustrated in FIG. 10, with parts ofthat plane 51 (or planes parallel to plane 51) being marked at 51A (inthe vicinity of the constraint member 50) and 51B (in the vicinity ofthe encoder scale 50).

In this embodiment, the elongate member 66 is generally square in crosssection, and is arranged relative to the constraint member 50 such thattwo of its four sides are substantially parallel with the plane 51 andthe other two sides are substantially perpendicular to the plane 51. Theencoder scale 10 is affixed to a surface that is parallel to the plane51. Therefore, in the front view of the constraint member 50 as shown inFIG. 9A, the encoder scale 10 is affixed to a side surface of theelongate member 66, while in the side view as shown in FIG. 9D and FIG.10 the encoder scale 10 is affixed to a front surface of the elongatemember 66. Furthermore, the plane 51 is arranged such that it isparallel to the direction 99 of gravity (i.e. so that the direction 99lies in the plane 51). The rotation axis 59A of the rotary joint 59 isalso arranged parallel to the direction 99 of gravity. The encoder scale10 is spaced away from the longitudinal axis 94 of the elongate member66 in a direction 90, 92A perpendicular to the plane 51, 51B defined bythe constraint member 50.

In operation of this embodiment of the present invention, the constraintmember 50 is adapted to constrain motion of the elongate member 66relative to the plane 51 defined by the constraint member 50 when theconstraint member 50 is attached to the elongate member 66 and to afurther member of the coordinate positioning machine 100 (in thisembodiment the further member is in the form of the support block 22).In doing so, the constraint member 50 is able to prevent rotation of theencoder scale 10 away from a desired orientation, parallel with gravity,whilst still allowing changes in the angle between the elongate member66 and the upper and lower platforms 20, 40 which are required to allowthe lower platform 40 to move relative to the upper platform 20.

The constraint member 50 operates by maintaining a substantiallyconstant angle between the plane 51 defined by the constraint member 50and a vector 92A, 92B that is transverse to a longitudinal axis 94 ofthe elongate member 66 (and which moves with the elongate member 66).For example, vector 92A is perpendicular to the longitudinal axis 94 andis also perpendicular to the surface of the elongate member 66 on whichthe encoder scale 10 is attached. Since the attachment surface for theencoder scale 10 is parallel with plane 51B, the vector 92A is alsoperpendicular to the plane 51B defined by the constraint 50. This isillustrated in both FIG. 10 and FIG. 11.

Now consider all possible movements of the elongate member 66 that arepermitted by the constraint member 50, and the effect these have on theangle between the plane 51 defined by the constraint member 50 and thevector 92A. The constraint member 50 prevents rotation about a firstrotational degree of freedom 70, which is rotation about thelongitudinal axis 94. The constraint member 50 also prevents rotationabout a second rotational degree of freedom 72, which is rotation of theelongate member 66 out of the plane 51. On the other hand, theconstraint member 50 allows rotation about a third rotational degree offreedom 74, which is rotation of the elongate member 66 within the plane51.

By constraining movement to the third rotational degree of freedom 74,within or parallel to the plane 51 defined by the constraint member 50,the angle between the plane 51 and the vector 92A is maintainedsubstantially constant. Any change in that angle would lead to theencoder scale 50 rotating out of the plane 51B, and would subject theencoder scale to undesirable types of distortion described above withreference to FIG. 3.

A similar analysis can be made in respect of other vectors that aretransverse to the longitudinal axis 94, such as vector 92B which isperpendicular to an adjacent surface of the elongate member 66; theangle between vector 92B and the plane 51 remains substantially constant(zero) for all movements of the elongate member 66. The vector underconsideration need not be perpendicular to the longitudinal axis 94,merely transverse to the longitudinal axis 94, but in all cases theangle between the chosen vector and the plane 51 remains substantiallyconstant.

Constraining the angle of the traverse vector 92A, 92B with respect tothe plane 51 defined by the constraint member 50 effectively locks theelongate member 66 against any rotation around its longitudinal axis 94,whilst still allowing angular movement between the elongate member 66with respect to the upper and lower platforms 20, 40.

With the constraint member 50 being attached to the upper platform 20via an attachment feature in the form of rotary joint 59, rotation ofthe plane 51 is enabled about attachment axis 59A defined by the rotaryjoint 59. This allows a generous freedom of movement to the extendableleg assemblies, and thereby also the moving (lower) platform 40, whilststill maintaining the plane 51 (and therefore encoder scale 10) inalignment with gravity 99. This ensures that the problem explained abovewith reference to FIG. 3 is overcome.

FIG. 12 is a schematic illustration of the elongate member 66 of theextendible leg assembly 60, showing in more detail how the scale 10interacts with a scale reader. In particular, FIG. 12 illustrates howthe scale 10 interacts (or cooperates) at a measurement location M witha scale reader 11 to provide a measurement relating to a length of theelongate member 66 (or equivalently of the extendable leg assembly 60).Such a measurement is used in this example to determine the position ofthe probe component 14 in the working volume.

The elongate member 66 comprises first and second elongate members 63and 65 which move relative to one another when the extendable legassembly 60 changes length. The first and second members 63, 65 havetheir respective longitudinal axes arranged substantially in line withone another, and slide over or past one another as the extendable legassembly 60 extends and retracts. In the example shown in FIG. 12, thefirst member 63 slides telescopically inside the second member 65. Thescale 10 is provided on or by the first member 63, with the scale reader11 being provided on or by the second member 65.

The extendible leg assembly 60 is supported at first and secondpositions P1 and P2, which in this example are first and second ends ofthe extendible leg assembly 60, with the measurement location M beingbetween the first and second positions P1 and P2. With this arrangement,the extendible leg assembly 60 is substantially unsupported (orself-supporting) at the measurement location M, so that it is subject asa result to gravitationally-induced bending at the measurement locationM (and prone to the technical problems described above with reference toFIG. 3 were it not for the present invention).

The measurement location M is spaced apart from both of the first andsecond positions P1 and P2, which is to be distinguished from acoordinate positioning device which provides support at or in thevicinity of the measurement location (and which therefore does notsuffer from the sorts of problems described with reference to FIG. 3).

The spacing between the measurement location M and at least one of thefirst and second positions P1, P2 varies as the probe component 14 movesaround the working volume (and as the length of the extendable legassembly 60 varies). FIGS. 12A, 12B and 12C show the extendible legassembly 60 with three different respective extensions, for threecorresponding positions of the probe component 14 in the working volume.

In FIG. 12A, the extendible leg assembly 60 is fully extended, or atleast as fully extended as the scale 10 and scale reader 11 will allow.In this respect, the scale reader 11 must be positioned over the scale10 in order for the scale 10 to be readable and for a measurement to betaken. A spacing S1 between the measurement location M and the firstposition P1 is roughly equal to a spacing S2 between the measurementlocation M and the second position P2.

In FIG. 12B, the extendible leg assembly 60 is retracted somewhat, withthe scale reader 11 having passed over around half of the length of thescale (or with the scale 10 having passed under the scale reader 11,depending on which end is moveable; both may be moveable too). Theextendible leg assembly 60 is around half-extended. The spacing S2 isunchanged, since the measurement location M is determined by theposition of the scale reader 11, which is affixed to the second member65. The spacing S2 between the measurement location M and the secondposition P2 has reduced.

In FIG. 12C, the extendible leg assembly 60 is retracted further, to afully retracted state; there is no further travel possible whilst stillensuring that the scale 10 and scale reader 11 are able to cooperatecorrectly to produce a measurement. Again, the spacing S2 is unchanged,while the spacing S2 has reduced further.

It can be seen that, particularly for the fully-extended configurationof FIG. 12A, there is a significant separation between the measurementlocation M and one or both positions P1 and P2.

For the fully-extended configuration of FIG. 12A, the measurementlocation M is spaced apart from both of the first and second positionsP1 and P2 by around ten times a width ‘W’ of the elongate member 66, andthe lack of support in the vicinity of the measurement location Mresults in noticeable bending and a noticeable effect on the metrologymeasurements from the encoder 10, 11. Expressed differently, themeasurement location M is spaced apart from both of the first and secondpositions P1 and P2 by approximately half of the overall length (S1+S2)of the extendible leg assembly 60. As the extendible leg assembly 60retracts, the spacing 51 reduces, but even in the fully-retractedconfiguration of FIG. 12C the measurement location M is spaced apartfrom both of the first and second positions P1 and P2 by at leastapproximately a third of the overall length (S1+S2) of the extendibleleg assembly 60 (which is the smaller separation S1). It will of coursebe appreciated that the example of FIG. 12 is purely illustrative, andthe range of movement and the relative spacings will vary fromembodiment to embodiment. What is significant is that there theextendible leg assembly 60 is self-supporting in the vicinity of themeasurement location M.

Although it is described above that the encoder scale 10 is affixed tothe elongate member 66, the encoder scale could also form part of or bedefined by the elongate member 66, for example formed directly in asurface of the elongate member 66.

It is also to be appreciated that the benefit of the present inventionis not limited to the use of an encoder scale 10 applied to the elongatemember 66 as described above. The invention would find benefit in anyapplication that incorporates a metrology element that is potentiallyaffected in the same way as described above. It is particularlybeneficial where, like an encoder scale, the metrology element issubstantially planar and is arranged substantially parallel to the planedefined by the constraint member 50.

Furthermore, it is not essential that all extendable leg assemblies inthe coordinate positioning machine are provided (or associated) with aconstraint member according to an embodiment of the invention; somebenefit would be obtained even with one such extendable leg assemblybeing so provided, though clearly it is beneficial to have more thanone, preferably all, provided (or associated) with a constraint member.

It has been described that the extendable leg assembly varies in lengthas the component moves around the working volume, and it is thisvariation in length that is used to position the component within theworking volume (i.e. either measure the position of the component in theworking volume, or set the position of the component in the workingvolume, or both). An embodiment of the present invention is to bedistinguished from a non-Cartesian positioning device having afixed-length arm that is moved through a fixed length-measuring element,with an encoder scale provided on the fixed-length arm for providing anindication of the position of the component.

Furthermore, although the predetermined part, whose orientation relativeto gravity is carefully controlled in an embodiment of the presentinvention, is described above as being a length-measuring part, thepresent invention is applicable to other types of part that may besensitive to orientation relative to gravity. In a differentapplication, it is therefore conceivable that a property of theextendable leg assembly other than length is measured or otherwisedetermined by or using the predetermined part.

It should also be emphasised that the present invention is not limitedto a constraint member 50 in the form of a multi-part hinge asillustrated, and other possible arrangements would be apparent to theskilled person. For example, another option for the constraint member 50would be a single piece of plastic with formed thinned section creatingthe hinges. Also, with a constraint member of the type as illustrated inthe appended drawings, there could be more or fewer joints provided thanis illustrated.

In the above-described embodiment, the constraint member 50 is attachedbetween the elongate member 66 and the support block 22 of thecoordinate positioning machine 100, so that movement of the elongatemember 66 is constrained relative to the coordinate positioning machine100, with the constrained motion being defined by the constraint member50 and its attachment to the support block 22 via the rotating joint 59.The support block 22 acts as an attachment feature and constitutes a‘further member’ of the coordinate positioning machine 100. Theconstraint member 50 effectively ties the elongate member 66 to thefurther member to constrain motion of the elongate member 66 relative tothe further member.

It is to be noted that it is not essential that the constraint membersare provided at the connection between the extendable leg assemblies 60and the upper platform 20, and could equally be provided instead at theconnection between the extendable leg assemblies 60 and the lowerplatform 40, or both. There could even be a mixture of such arrangementsamongst the extendable leg assemblies 60.

Also, the constraint member need not form a single assembly such as thehinged constraint member 50 described above, but may in fact consist oftwo or more separate assemblies that are each attached to the elongatemember 66 and the further member in order to provide the requiredconstraint.

It will be appreciated that other types of constraint member aresuitable for providing a predetermined part (e.g. the encoder scale 10)of the extendable leg assembly 60 with substantially a same orientationrelative to gravity for a same position of the component as thecomponent moves around the working volume. For example, FIG. 13illustrates a constraint member in the form of a weight 501 that isattached to and hangs from a lower surface of the upper tube 62, pulledin direction 99 by gravity. A point of attachment 503 for the weight 501to the tube 62 is spaced away from the longitudinal axis 94 in adirection substantially parallel to the surface of the elongate member66 on which the encoder scale 10 is affixed, and accordingly the weight501 will act to maintain the encoder scale 10 in a fixed orientationrelative to gravity (aligned with gravity).

The FIG. 13 embodiment shares the feature of the previously-describedembodiment that the constraint member (weight) 501 defines a plane 51,and that rotation of the elongate member 66 (with encoder scale 10) isconstrained relative to the plane 51. The plane 51 in this embodiment isdefined by the position of the hanging weight 501 and the two balljoints at either end of the elongate member 66, or alternatively by theweight 501, the attachment point 503 and either of the ends of theelongate member 66. Operation of the constraint member 501 keeps thevector 92A (perpendicular to the surface of the elongate member 66 onwhich the encoder scale 10 is attached) aligned with vector 90(perpendicular to the plane 51) and therefore keeps the encoder scale 10substantially aligned with gravity.

FIGS. 14 and 15 provide a schematic illustration of the use of aconstraint member in another embodiment of the present invention. Thisembodiment is useful where absolute accuracy is not the primaryrequirement for the machine but where repeatability is still veryimportant. In this embodiment, the constraint member ensuresrepeatability by preventing the struts rotating without constraint abouttheir primary (longitudinal) axis, so that a predetermined(orientation-sensitive) part of the extendable leg assembly hassubstantially a same orientation relative to gravity for a same positionof the component within the working volume.

As in the above-described embodiment, the FIG. 14 embodiment provides aconstraint member associated with the extendable leg assembly forproviding a predetermined part of the extendable leg assembly withsubstantially a same orientation relative to gravity for a same positionof the component in the working volume. The constraint member of FIG. 14is also adapted to constrain rotation of the elongate member relative toa plane defined by the constraint member when the constraint member isattached to the elongate member and to the further member.

The constraint member 510 illustrated in FIG. 14 is generally of asimilar type to the constraint member 50 illustrated in previousfigures, and like reference numerals refer to like parts. However, theFIG. 14 embodiment differs from the previous embodiment in that in theFIG. 14 embodiment the ‘further member’ is in fact the elongate member65 of another extendable leg assembly. The constraint member 510 of FIG.14 is used effectively to tie an elongate member 66-1 of a firstextendable leg assembly to an elongate member 66-2 of a second,adjacent, extendable leg assembly. The constraint member 510 is commonor shared between the two elongate members 66-1, 66-2. As with theprevious embodiment, the constraint member 510 of FIG. 14 defines aplane 51, but with the FIG. 14 embodiment the plane 51 defined by theconstraint member 510 is substantially parallel with a plane defined bythe two elongate members 66-1, 66-2, rather than being aligned withgravity. It is to be emphasised that the elongate members 66-1, 66-2 areonly illustrated schematically in FIGS. 14 to 17, and the drawings arenot intended to illustrate or imply any particular construction methodor material used to form the elongate members 66-1, 66-2, nor anyparticular cross-sectional shape.

Other than the constraint member 510 being attached in use between twoelongate members 66-1, 66-2 of the coordinate positioning machine, themain difference between the constraint member 510 of FIG. 14 and theconstraint member 50 of previous figures is that the constraint member510 of FIG. 14 is connected to the further member 66-2 via a fixedconnection, rather than the rotary connection 59 of FIGS. 6 to 8, anddoes not therefore allow rotation of the plane relative to the furthermember 66-2, or indeed vice versa. The constraint member 510 of FIG. 14therefore acts to prevent rotation of both elongate members 66-1, 66-2about their respective longitudinal axes, whilst allowing angularmovement between the elongate members 66-1, 66-2.

The embodiment of FIG. 14 is provides a constraint that ensures arepeatable but varying orientation for a predetermined part of theextendable leg assembly, and in particular an encoder scale. Althoughnot shown in FIG. 14, the encoder scale would be arranged in relation tothe elongate members 66-1, 66-2 in a similar as described with referenceto FIG. 12. Although the orientation of the encoder scale relative togravity will vary as the moveable component (e.g. probe) is moved aroundthe working volume, the orientation will be the same for each visit tothe same position in the working volume. When the machine is used as acomparator, this variation does not matter, because the same amount andtype of bending will have been present for the same position whenprobing the reference part. It matters more that the amount and type ofbending is repeatable, from probing the reference part to probing theactual part, and when moving around the working volume in general.

The presence of the constraint member is also useful where there is aheavy component attached to the outside of one or both of the elongatemembers 66-1, 66-2, such as a motor for extending the leg assembly. Theforce of gravity acting on the component will tend to rotate theelongate member 66-1, 66-2 around its longitudinal axis, i.e. relativeto the plane defined by the constraint member 510.

Even in the absence of a metrology element such as an encoder scale thatmight be affected by such rotation, such rotation can still lead toproblems. For example, a joint may be shared between two leg assembliessuch that the leg assemblies are situated in close proximity at thejoint; in this situation, any rotation of an elongate member about itslongitudinal axis may cause the adjacent leg assemblies to clash withone another, which in turn can cause the leg assembly to lift slightlyoff the joint. This is likely to lead to measurement errors, or couldeven cause a leg assembly to come off the joint completely.

The use of a constraint member 510 in such a situation prevents suchrotation about the longitudinal axis, or at least reduces such rotationto a desirable extent so that the risk of the ends of adjacent legassemblies clashing is minimal.

FIGS. 16A and 16B illustrate an alternative to the constraint member ofFIGS. 14 and 15. The constraint member 520 of FIGS. 16A and 16Bcomprises first and second constraint arms 522, 524 and first and secondattachments 526, 528. The first and second constraint arms 522, 524 aresubstantially L-shaped and are connected slidably together by the firstand second attachments 526, 528 (e.g. low-friction bushes) to form agenerally rectangular shape. Opposite sides of the rectangular shapeformed by the first and second constraint arms 522, 524 are heldrespectively by elongate members 66-1, 66-2, with the rectangular shapebeing free only to pivot or rotate about the longitudinal axis of theheld side of the arm.

Due to the slidable connection between the first and second constraintarms 522, 524, the rectangular shape is extendible to allow angularmovement between the elongate members 66-1, 66-2, but the rectangularshape braces either elongate member 66-1, 66-2 against rotation abouttheir respective longitudinal axes. The constraint member 520 isarranged such that a plane defined by the rectangular shape formed bythe constraint member 520 is substantially perpendicular to the planedefined by the elongate members 66-1, 66-2.

Therefore, as with the constraint member of FIGS. 14 and 15, theconstraint member 520 of FIGS. 16A and 16B defines a plane 51 (see FIG.16B), with the plane 51 also being substantially parallel with the planedefined by the two elongate members 66-1, 66-2. The presence of theconstraint member 520 causes rotation of each elongate member 66-1, 66-2to be constrained relative to the plane 51 defined by the constraintmember 520.

FIGS. 17A and 17B illustrate another alternative to the constraintmember of FIGS. 14 and 15. The constraint member 530 of FIGS. 17A and17B comprises first and second (e.g. metal) constraint plates 532, 534.The first constraint plate 532 is fixed to one side of the secondelongate member 66-2, and is magnetically preloaded into contact with afirst bearing 536 on the opposite (first) elongate member 66-1.Similarly, the second constraint plate 534 is fixed to the other side ofthe first elongate member 66-1, and is magnetically preloaded intocontact with a second bearing 538 on the opposite (second) elongatemember 66-2.

Therefore the pair of elongate members 66-1, 66-2 has two plates 532,534 and bearing mechanisms 536, 538 which work to resist twisting of theelongate members 66-1, 66-2 (rotation around their respectivelongitudinal axes) and hold them parallel. As with the constraint memberof FIGS. 14 and 15, the constraint member 530 defines a plane 51 (seeFIG. 17B) that is substantially parallel with a plane defined by the twoelongate members 66-1, 66-2. The presence of the constraint member 530causes rotation of each elongate member 66-1, 66-2 to be constrainedrelative to the plane 51 defined by the constraint member 530.

Reference has previously been made to WO 2007/144573, in which acoordinate positioning machine is provided with a metrology frame thatis separate from the thrust frame. As mentioned, the separation of theload-bearing structure from the metrology structure applies to each ofthe six extendable legs, with each extendable leg being provided withhave a load-bearing structure and a metrology structure, and with themetrology structure being mechanically isolated from the load-bearingstructure. In such a case, a constraint member as described herein needonly be associated with the metrology structure of each extendable legassembly, for controlling the orientation of the metrology structure(for example, relative to gravity), though a constraint member canoptionally also be associated with the load-bearing structure of eachextendable leg assembly.

Although the non-Cartesian coordinate positioning machine illustrated inthe appended drawings has six extendable leg assemblies, a non-Cartesiancoordinate positioning machine embodying the present invention is ofcourse not limited to having six extendable leg assemblies, with thenumber and configuration of extendable leg assemblies being determinedby the application concerned.

Although an embodiment of the invention has been described mainly in thecontext of a coordinate measuring machine and a comparator, theinvention is applicable more generally to any type of coordinatepositioning machine, such as scanning machines, machine tools, robots,positioning devices (e.g. for optical components), prototypemanufacturing machines and various other uses.

The invention claimed is:
 1. A non-Cartesian coordinate positioningmachine comprising an extendable leg assembly for positioning acomponent within a working volume of the machine, an encoder scalearranged on or forming part of or being defined by an elongate member ofthe extendable leg assembly, and a constraint member associated with theextendable leg assembly for providing a plane defined by the encoderscale with a substantially constant orientation relative to gravity asthe component is moved around the working volume.
 2. A machine asclaimed in claim 1, wherein the constraint member is arranged tomaintain the plane substantially aligned with gravity, such that anormal to the plane is maintained substantially normal to gravity.
 3. Amachine as claimed in claim 1, wherein the encoder scale is subject tostresses caused by bending of the elongate member due to gravity.
 4. Amachine as claimed in claim 1, wherein the constraint member is adaptedto constrain rotation of the elongate member around its longitudinalaxis.
 5. A machine as claimed in claim 1, wherein the encoder scale isarranged to interact at a measurement location with a further part ofthe machine to provide a measurement signal.
 6. A machine as claimed inclaim 5, wherein the extendable leg assembly is supported by at leastone support, and wherein the measurement location is spaced apart fromthe or each support.
 7. A machine as claimed in claim 1, wherein theextendable leg assembly comprises first and second elongate memberswhich are arranged to move relative to one another when the extendableleg assembly changes length.
 8. A machine as claimed in claim 7, whereinthe encoder scale is arranged to interact at a measurement location witha further part of the machine to provide a measurement signal, andwherein the encoder scale is provided by or on the first elongate memberand the further part is provided by or on the second elongate member. 9.A machine as claimed in claim 1 wherein the encoder scale has asubstantially planar surface defining the plane of the encoder scale.10. A machine as claimed in claim 1, wherein the constraint member isadapted to constrain rotation of the encoder scale relative to a planedefined by the constraint member.
 11. A machine as claimed in claim 10,wherein the constraint member is attachable to the elongate member andto an attachment feature of the coordinate positioning machine, andwherein the constraint member is adapted to constrain rotation of theencoder scale relative to the plane defined by the constraint memberwhen the constraint member is attached to the extendable leg assemblyand to the attachment feature.
 12. A machine as claimed claim 10,wherein the constraint member is adapted to allow rotation of the planedefined by the constraint member about an attachment axis defined by theattachment feature.
 13. A machine as claimed in claim 12, wherein theattachment axis is parallel to the plane defined by the constraintmember and/or wherein the constraint member is arranged in the machinewith the attachment axis being substantially aligned with gravity.
 14. Amachine as claimed in claim 10, wherein the constraint member isarranged in the machine with the plane defined by the constraint memberbeing substantially aligned with gravity.
 15. A machine as claimed inclaim 10, wherein the encoder scale is one or both of: (a) spaced awayfrom a longitudinal axis of the extendible leg assembly in a directionperpendicular to the plane defined by the constraint member; and (b)arranged substantially parallel to the plane defined by the constraintmember.
 16. A machine as claimed in claim 1, wherein the componentcomprises a measurement probe and wherein the machine is a coordinatemeasuring machine or a comparator.
 17. A machine as claimed in claim 1,wherein the constraint member comprises a plurality of hinged sectionswith substantially parallel hinge axes.
 18. A machine as claimed inclaim 1, comprising a plurality of such extendable leg assemblies andsuch a constraint associated with each of the plurality of legassemblies.
 19. A constraint member for a non-Cartesian coordinatepositioning machine having an extendable leg assembly for positioning acomponent within a working volume of the machine and an encoder scalearranged on or forming part of or being defined by an elongate member ofthe extendable leg assembly, wherein the constraint member is adapted toprovide a plane defined by the encoder scale with a substantiallyconstant orientation relative to gravity as the component is movedaround the working volume.
 20. An extendable leg assembly for anon-Cartesian coordinate positioning machine, the extendable legassembly comprising a constraint member as claimed in claim
 19. 21. Anextendable leg assembly for a non-Cartesian coordinate positioningmachine, the extendable leg assembly comprising an elongate member, withan end of the elongate member being provided with a bearing arrangementhaving three contact points, or at least substantially point-likecontact surfaces or areas, for bearing against an at least partspherical bearing surface provided on the machine, where a plane definedby the contact points or surfaces or areas is substantiallyperpendicular to a longitudinal axis of the elongate member.
 22. Anextendable leg assembly as claimed in claim 21, wherein the bearingarrangement provides a kinematic or at least pseudo-kinematic couplingbetween the elongate member and the machine.
 23. An extendable legassembly as claimed in claim 21, wherein the at least part sphericalbearing surface is provided by a ball, or part thereof, fixed inrelation to the machine.
 24. An extendable leg assembly as claimed inclaim 21, wherein such a bearing arrangement is provided at both ends ofthe elongate member.
 25. An extendable leg assembly as claimed in claim21 wherein the three contact points or surfaces or areas are provided bythree at least part spherical surfaces, such as three balls or partsthereof, each of which may be smaller than the at least part sphericalsurface associated with the machine.
 26. An extendable leg assembly asclaimed in claim 21, wherein the three contact points or surfaces orareas are provided by a kinematic cup or cone.
 27. A non-Cartesiancoordinate positioning machine comprising an extendable leg assembly asclaimed in claim 21 for positioning a component within a working volumeof the machine.